The present invention relates to semiconductor structures and devices formed in wide bandgap materials such as silicon carbide and the Group III nitrides.
The performance capabilities of semiconductor devices fundamentally depend upon the inherent properties of the semiconductor materials from which they are made, as well as the extent to which these materials can be incorporated in a useful device structure. Semiconductor devices also depend upon the manner in which various semiconductor materials are formed and arranged with respect to one another.
One of the characteristics of semiconductor materials is their bandgap; i.e., the energy difference between the valence band of electrons and the conductance band. The size of the material's bandgap provides fundamental limitations upon—or possibilities for—device structures and performances.
As one example, microwave systems—typical examples of which include cellular communications systems—commonly use solid state transistors as amplifiers and oscillators. As such systems expand in subscribers and desired (or required) capacity, interest in increasing their operating frequency and power has grown correspondingly. Higher frequency signals can carry more information (bandwidth), allow for smaller antennas with very high gain, and provide systems such as radar with improved resolution. Because a larger bandgap can accommodate wider bandwidth signals, wide bandgap materials such as silicon carbide (SiC) and the Group III (Ga, Al, In) nitrides have been, and continued to be, materials of significant interest for high frequency devices.
Higher bandgap materials also offer the potential for higher power capabilities (as compared to similar structures made from smaller bandgap materials) as well as the potential for emitting light at higher frequencies; e.g., the green, blue, violet and ultraviolet portions of the electromagnetic spectrum.
Devices fabricated from silicon carbide are typically passivated with an oxide layer, such as SiO2, to protect the exposed SiC surfaces of the device, or for other reasons, or both. The interface between SiC and SiO2, however, may be insufficient to obtain a high surface mobility of electrons. More specifically, the interface between SiC and SiO2 conventionally exhibits a high density of interface states, which may reduce surface electron mobility and introduce carrier traps, which in turn reduces the desired performance characteristics of devices such as (but not limited to) metal-oxide-semiconductor field-effect transistors (MOSFETs).
Accordingly, in many circumstances semiconductor devices, including those that include oxidation layers, also incorporate one or more layers of silicon nitride to improve the resulting electronic properties (e.g., U.S. Pat. No. 6,246,076). Silicon nitride also provides an environmental barrier that the oxide fails to provide, or without which, would allow the environment to degrade the structure and operation of the device, regardless of whether the device includes an oxide layer. As an environmental barrier, silicon nitride is preferred over silicon dioxide because it forms a better seal over the device, preventing contaminants such as water from reaching the epitaxial layers of the device and from causing degradation. Silicon nitride may also be used to form layers that transmit light generated within an LED.
The dense structure of silicon nitride does not provide the open channels found in oxide structures; thus, nitride is widely employed in electronics as a barrier material. In particular, hydrogen diffuses slowly in a densified nitride film, and other small positive ions (Na+ or K+) are effectively blocked by thin nitride layers. Because oxygen diffuses very slowly through nitride, deposited nitride can prevent oxidation of underlying silicon.
Nevertheless, nitrides deposited using chemical vapor deposition (often plasma enhanced chemical vapor deposition, “PECVD”) almost always contain hydrogen, typically much more than in the comparable oxide films. The source of the hydrogen is the silane precursor and also the ammonia employed in many CVD schemes. An amorphous, but constrained, film such as silicon nitride can discourage the atoms from occupying positions that fill the valences of each silicon and nitrogen atom. Thus, many broken bonds tend to be present. These bonds are readily occupied by hydrogen atoms. Thus, conventional plasma nitrides can have as much as 20 atomic percent hydrogen, bonded both to the Si and N atoms; thermal nitrides still have several percent hydrogen even after high-temperature anneals.
Additionally, hydrogen can passivate Mg-acceptors in a GaN-based semiconductor. Although the precise mechanism is not completely understood, when silicon nitride is deposited by means of PECVD at a deposition temperature in excess of 200° C., hydrogen in the film can diffuse through thin ohmic contacts or other layers and into nearby Group III nitride layers, causing them to become passivated in a region close to their surface. That is, in a region near the surface, a substantial number of acceptor ions are rendered neutral by the introduction of hydrogen in the film. Accordingly, an interface between an ohmic contact and a nitride material is degraded, and the contact metal does not exhibit ideal ohmic characteristics. This can result in an increase in forward voltage (Vf degradation) in the device. Essentially, the device will behave as though the interface between a metal and a Group III nitride contact layer forms a Schottky contact instead of an ohmic contact.
Because nitride passivation layers are often used in conjunction with oxide layers, the hydrogen can migrate to the oxide layers. In turn, hydrogen in oxide films on SiC has been shown to alter the interface Fermi level and encourage a state of surface accumulation. Any resulting accumulation layer produces a charge layer that alters the device capacitance and exhibits a drift with a long time constant caused by the mobility of the hydrogen in the film.
Accordingly, although oxide and nitride layers offer certain advantages, they also raise certain problems that can limit or degrade device performance.